Displays utilizing two dimensional arrays, or matrices, of pixels each containing one or more light emitting devices are very popular in the electronic field and especially in portable electronic and communication devices, because large amounts of data and pictures can be transmitted very rapidly and to virtually any location.
Light emitting device (LED) arrays are becoming more popular as an image source in both direct view and virtual image displays. One reason for this is the fact that LEDs are capable of generating relatively high amounts of light (high luminance), which means that displays incorporating LED arrays can be used in a greater variety of ambient conditions. For example, reflective LCDs can only be used in high ambient light conditions because they derive their light from the ambient light, i.e. the ambient light is reflected by the LCDs. Some transflective LCDs are designed to operate in a transmissive mode and incorporate a backlighting arrangement for use when ambient light is insufficient. In addition, transflective displays have a certain visual aspect and some users prefer a bright emissive display. However, these types of displays are generally too large for practical use in very small devices.
Also, organic light emitting diodes or organic electroluminescent devices and arrays thereof (hereinafter included as LEDs) are emerging as a potentially viable design choice for use in small products, especially small portable electronic devices, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. Organic LED arrays are capable of generating sufficient light for use in displays under a variety of ambient light conditions (from little or no ambient light to bright ambient light). Further, organic LEDs can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that organic LED arrays can be fabricated in a variety of sizes. Also, organic LEDs have the added advantage that their emissive operation provides a very wide viewing angle.
A major disadvantage in the use of LED arrays is the complexity of the manufacturing process. In passive arrays, for example, each LED requires two contacts, generally an anode and a cathode, connected to column and row buses. The arrays are generally manufactured by depositing materials on a supporting substrate and addressing/driver connections to row and column buses are made around the edges, which means that additional power (current for organic LEDs) is required for LEDs farther from the source. Another problem complicating the fabrication of LED arrays is the fact that the fill factor (the ratio of the emitting surface to the total surface) is limited by the data addressing metal lines. For example, in LED passive arrays, as the number of pixels in an array is increased to increase the resolution, the column metal line width increases to prevent electromigration effect. Thus, if the LED emitting area is fixed, the fill factor decreases as the array resolution increases, e.g. 1/8 Video Graphics Array (VGA) has a 25% fill factor, 1/4 VGA has a 16% fill factor, and VGA has an 8% fill factor. Further, the pixel pitch increases as the array resolution increases, e.g. 1/8 Video Graphics Array (VGA) has a 20 .mu.m pitch, 1/4 VGA has a 25 .mu.m pitch, and VGA has a 35 .mu.m pitch. Also, the driving current increases as array resolution increases, e.g. 1/8 Video Graphics Array (VGA) has a driving current of 7.2 mA, 1/4 VGA has a driving current of 12 mA, and VGA has a driving current of 24 mA.
Accordingly, it would be beneficial to provide an LED array and driving apparatus which overcomes these problems.
It is a purpose of the present invention to provide a new and improved active LED array and driving apparatus.
It is also purpose of the present invention to provide a new and improved active LED array and driving apparatus which requires less manufacturing steps and is simpler to manufacture.
It is another purpose of the present invention to provide a new and improved active LED array and driving apparatus with reduced metal line widths and a reduction in addressing current
It is another purpose of the present invention to provide a new and improved active LED array and driving apparatus with a reduced pitch and a higher fill factor.
It is still another purpose of the present invention to provide a new and improved active LED array and driving apparatus which is easier and less expensive to fabricate and use.
It is a further purpose of the present invention to provide a new and improved active LED array and driving apparatus in which the LED, active circuits and drivers are integrated onto a single substrate.